The invention relates generally to demand side electricity resources, and more particularly to a method for integrating demand response into optimal dispatch algorithms for electric power systems.
Existing demand response programs do not capture the impact of shifting/deferring load from one time period to another. When demand is dispatched, the load agrees to reduce consumption for the specified hour(s) according to the demand bid. Once the curtailment period expires, the current programs typically assume that the demand will then return to its “normal” or uncurtailed, baseline state (or at least fail to take the rebound effect into account). In reality, however, many types of demand will actually consume more than their “normal” state following a curtailment event. This increase in subsequent periods can be called a rebound effect.
For deferred/shifted loads, consumption is delayed; and so it should be expected that subsequent periods will see an increase in demand. For some applications, demand is not deferred, but instead foregone completely. Lighting, for example, cannot typically be deferred; and so if turning off lights is part of a curtailment, that load will not rebound. Delaying an industrial process on the other hand, will clearly result in an increase in subsequent load.
For small demand response programs or those with little load deferral, optimal dispatch may safely ignore the rebound effect with little efficiency loss. As demand response programs increase in size and more participation occurs via load shifting, the impacts of the rebound effect will result in greater efficiency losses, and the potential for secondary peaks being created.
Electricity demand can be categorized by demand for power and demand for energy. Demand for power is instantaneous, while demand for energy is not. Energy based demand can be utilized as a storage mechanism for electric power, allowing for short-term substitution between periods. In addition, the services provided by equipment which demands power rather than energy are not time dependent in many cases.
Classifying electricity demand by end use enables an estimation of both the magnitude of and time scales associated with potential demand bidding blocks. Residential demand can be broken down into three categories: Core Demand, Deferrable Demand, and Thermal Demand.
Core demand is defined as the segment of load that is dependent upon real time power delivery. This includes lighting, televisions, computers (not including laptops with rechargeable batteries), and small appliances. The indirect services provided by electricity in these cases are available only as long as power is flowing to the equipment/appliance providing the service. This segment of load is relatively inelastic in the short term because it cannot be deferred. Any curtailments or reductions in core demand will not typically be shifted to a later time period. Core demand has been estimated to account for approximately 36% of the total residential electricity demand.
A second category of electricity demand consists of deferrable load. Deferrable demand is somewhat of a hybrid between power based loads and energy based loads. Although the service provided by these loads depends on continuous power for a fixed period of time, the demand for the service itself is not instantaneous and often consumers may be indifferent to provision of the service within a range of time periods. Washers, dryers, dishwashers, and electric ovens are examples of appliances that have deferrable load. Consumers often are not concerned with the exact times that such appliances run, as long as it is within a certain interval. Consumers will often be willing to delay the use of these appliances for several hours without loss of consumer benefit from the service, especially if the delay can be automated by the use of timer controls. This presents an opportunity for deferring the power consumption by these appliances from peak to off peak time periods—especially if programmable controls are available to automate the deferral. Although these appliances typically make up a small portion of the total residential load due to their intermittent usage, they do consume significant amounts of power while running and therefore offer the potential for significant peak shaving whenever they can be shifted to off peak consumption. Slightly less than 10% of the total residential electricity demand has been estimated to consist of deferrable load. Commercial and industrial customers may also defer certain processes from a given time period to another.
The final category of demand is thermal demand, and it consists of air conditioners, refrigerators, water heaters, and electric space heaters. These provide service based on thermal transfer (heat or cooling). As such, consumers are indifferent to the actual time that this equipment runs, as long as the temperature remains within a certain range. By intelligently controlling consumption of energy based loads, the desired temperature band can be utilized as a thermal storage medium, and therefore as an indirect electricity storage method. The time scales for deferring thermal demand are typically on the order of 1-4 hours, thus creating a large potential for a rebound effect when dispatched as part of a demand response program.
Thermal loads can also be pre-heated or pre-cooled during low priced periods or activated/deactivated on a shorter time period in response to changes in system conditions. Thermal load has been estimated to account for nearly 50% of total residential consumption. It is also a significant portion of commercial and industrial demand. This represents a very large potential for load shifting in order to reduce peak demand by utilizing thermal storage. Air conditioning has been estimated to account for over 20% of household electricity usage in the United States. Air conditioning load is also highly peak coincident, since summer peaks are almost entirely caused by air conditioning load. Residential and commercial air conditioning load have been estimated to represent at least 30% of the summer peak electricity loads.
Core demand is instantaneous and inelastic. Core loads require continuous power in order to maintain their services and represent a little over one third of total residential demand. Deferrable loads require power to provide their services, but the demand for those services is not instantaneous. This presents the potential to defer the energy consumption in response to system conditions, such as price or contingencies. Deferrable load has been estimated to consist of less than 10% of total residential demand. The final category, thermal demand, consists of thermal based services that require energy rather than instantaneous power. Over 50% of residential demand can be classified as thermal demand.
It is likely that as demand response programs expand to include more residential customers, thermal and deferrable demand will be the predominant means of residential participation. Due to the nature of this type of demand, this will create a large potential for a rebound effect in subsequent hours after demand is dispatched. This will increase the need for a demand dispatch methodology that includes the rebound effect.
Many existing Demand Response programs have a limit to the number of times that a participating customer may be dispatched (i.e. called upon to respond) within a given time period. For example, a residential customer's air conditioning load may be reduced up to 10 times per year.
In view of the foregoing, it would be advantageous to provide a system and method for integrating demand response into optimal dispatch algorithms for electric power systems that includes the rebound effect. It would also be beneficial if the system and method were to include the opportunity cost of dispatching resources with a contractually limited number of dispatches in a given time period.